This invention relates to an attitude sensor, in particular a single axis solid state attitude sensor which may be rotated about its sensitive axis.
In a single axis attitude sensor, it is known to set a closed-end cylinder into a mode of vibration by applying an oscillatory signal to primary actuator(s) positioned around the side walls of the cylinder as is shown in FIG. 1. For example, the (primary) mode that is excited in the cylinder has a radial amplitude of cos2.theta.coswt (where w=the resonant frequency of the cos2.theta. mode) and the mode vibrates as shown in FIG. 2. In practice, it is necessary to derive a signal from the cylinder by means of an appropriate primary pick-off whose amplitude is dependant upon the amplitude of the mode. This derived signal can be fed back to the oscillator to adjust the oscillator output thus ensuring that the cylinder is maintained at the desired resonance (mode of vibration).
When the cylinder is rotated about its axis, the direction of the vibrating mode lags behind the cylinder, by an amount that corresponds to the excitation of a (secondary) mode whose radial dependence is sin2.theta.coswt. The amplitude of this mode can be measured using appropriate secondary pick-off(s). The resulting signal when demodulated with respect to the oscillator output, can be used as a measurement of the rotation rate that is applied to the cylinder.
The pick-off signal which measures the amplitude of the sin2.theta.coswt can also be used as shown in FIG. 3. Here the signal is amplified and fed back to secondary actuator(s) positioned around the cylinder to null the amplitude of the sin2.theta.coswt mode to approximately zero.
The amplitude of the signal fed back to the actuator is dependant upon the rotation rate applied to the cylinder and thus if the signal is demodulated with respect to the oscillator signal, the output of the demodulation is a measurement of the rotation rate. The measurement rotation rate obtained through feedback, in general, can be expected to be a more linear measurement of rotation rate than that obtained without feedback.
It is known to construct the cylinder of various materials, such as fused silica, Beryllium-Copper, PZT etc. Various drive and pick-off mechanisms have been proposed including, for example, piezo-electric, magnetic, electrostatic, thermal or resistive thermal devices. Other shaped resonators are known such as discs, open-ended cylinders, hemispheres, and other shells with azimuthal symmetry. Castellations have also been incorporated into some designs.
However, these designs all suffer from a common drawback, namely that a scale factor of the rate sensor is dependent upon the Q factor of the cylinder, the efficiency of the actuator, and the gain of the demodulators. Thus the scale factor is not constant as one would ideally require. Whilst feedback of the primary pick-off signal to the primary actuator can stabilise the amplitude of the primary mode, this solution does not eliminate the scale factor problem.
One reason why the scale factor accuracy problem is so important, can be understood by examining the following system. A rate sensor output is connected to an integrator whose output is the angle through which the cylinder is turned about its axis--ie, the system is a single-axis attitude sensor (or rate integrating gyroscope). If there is a 1% scale factor error, then the output will be in error by 3.6.degree. for each revolution of the cylinder no matter how fast the cylinder is turned. This error will be additive to the other errors arising from drifts and nonlinearity.
A conventional solution to the scale factor problem would be to place the rate sensor on a rotating platform and to use the output of the rate sensor to drive a motor such that the cylinder does not rotate when the system is moved. This solution would work, but would not have the advantages in reliability and manufacturability offered by the solid-state solution proposed by this invention.